As an animal there is only one thing you have to do. Further the species.

Why. To achieve what end? Just because we are genetically designed to enjoy sex, what objective value is there in procreating our species, as opposed to promoting other species or the planetary ecosystem? Your entire argument is circular.

We also don't live in a world where you can just have sex and move on.

False. You can and people do. It's called a one night stand for a reason.

What giegerwasright said is perfectly valid.

Except that it isn't. He claims that casual sex somehow produces or enables feminists, without providing a basis or logic for that claim. He is in essence male-slut shaming: Instead of "If you dress slutty, it's your fault you get raped" it's "if you have casual sex, it's your fault feminists exist and screw you over" which is nonsensical.